1 Field of the Invention
The present invention generally relates to a laser-based photoacoustic sensor for trace detection and differentiation of atmospheric NO and NO.sub.2. More particularly, the invention relates to a device and method for detecting NOx in real-time and in-situ, in which a photoacousitc sensor employs a tunable laser for continuously measuring and differentiating atmospheric concentrations of nitric oxide (NO) and nitrogen dioxide (NO.sub.2)
2 Description of the Prior Art
There is a growing interest in laser-based analytical techniques for remote or in-situ trace detection of NO and NO.sub.2. Much of this interest stems from concerns related to public health and the environment. These compounds play key roles in catalytic ozone destruction, and in acid rain and photochemical smog formation. In particular, NO and NO.sub.2 are hazardous pollutants emitted predominantly from motor vehicle exhaust and stationary sources, such as electrical utility generators and industrial boilers. The U.S. Federal Environmental Protection Agency has established a 25-ppm (parts per million) National Ambient Air Quality Standard threshold limit for NO with concentrated exposures not to exceed 100 ppm for fifteen minutes. NO.sub.2 is estimated to be thirty times more toxic than NO (see J. A. Last et al., "Ozone NO and NO.sub.2 Oxidation and Air Pollutants and More", Environ. Health Perspect. 102, Suppl. 10, 179 (1994)).
The detection of NO and NO.sub.2 is also important to laser photofragmentation and fragment-detection techniques being developed for the chemical analysis of propellants and explosives because they are generated in the photolysis of many energetic materials. In that regard, see J. B. Simeonsson and R. C. Sausa, "A Critical Review of Laser Photofragmentation/Fragment Detection Techniques for Gas-Phase Chemical Analysis", Applied Spectroscopy Reviews, 31 (1&2) p. 1, 1996).
Conventional methods for determining ambient concentrations of NO and NO.sub.2 include chemiluminescence and passive collection with subsequent wet chemical analysis. However, these methods are relatively slow (min-hrs), and have problems in discriminating between NO and NO.sub.2, particularly at low concentrations.
Laser-based methods for NO and NO.sub.2 detection are being implemented more frequently because they offer rapid and real-time monitoring capabilities with excellent sensitivity. These methods include laser-induced fluorescence (LIF), resonance-enhanced multiphoton ionization (REMPI), and laser photoacoustic spectroscopy (PA). For specific examples of LIF and REMPI detection, see the review article by J. B. Simeonsson and R. C. Sausa mentioned above, as well as the references cited therein. For examples of PA detection, see C. Williamson, R. Pastel and R. Sausa, "Detection of Ambient NO by Laser-Induced Photoacoustic Spectroscopy using A.sup.2 .SIGMA..sup.+ -X.sup.2 II (0,0) Transitions Near 226 nm", Applied Spectroscopy, 50(2)m p. 205, 1996; L. B. Kreyzek, N. D. Kenyon and C. K. N. Patel, Science, 177, 347 (1992); and A. Fried, Appl. Spect. 36, 562 (1982); and references cited therein. The activity described in the above-mentioned references centers on the detection of either NO or NO.sub.2, but not both simultaneously.
The detection and discrimination of NO and NO.sub.2 by a single, laser-based apparatus have been important and formidable analytical challenges. Part of the challenge stems from the fact that NO and NO.sub.2 absorb in different spectral regions. NO.sub.2 absorbs in the visible region, whereas NO absorbs in the ultraviolet region. NO.sub.2 predissociates at wavelengths of less than 400 nm, making ionization and LIF detection difficult. Although both NO and NO.sub.2 absorb in the infrared region, few lasers can be tuned in the region where both species absorb. In addition, H.sub.2 O is a major spectral interferant in the infrared region.
A device and process for detecting and discriminating NO and NO.sub.2 were recently reported (J. Simeonsson and R. Sausa, "Trace Analysis of NO.sub.2 in the Presence of NO by Laser Photofragmentation/Fragment Photoionization Spectrometry at Visible Wavelengths," applied Spectroscopy, Vol 50, Number 10, 1996). In that device and process, NO and NO.sub.2 molecules are differentiated spectrally by using a visible laser and a simple flow cell with miniature electrodes for ion detection. NO is detected by a REMPI process, whereas NO.sub.2 is detected by a laser photofragmentation/fragment ionization process. Limits of detection in the low parts per billion range were reported for NO and NO.sub.2.
Although these ionization techniques offer high sensitivity under many conditions, there are situations where these techniques are less suitable. In particular, they suffer from collisional quenching and non-resident background ionization at atmospheric pressure because of the multiphoton processes involved and the relatively high laser fluences required.
At present, there is a need for the development of a method and apparatus for the rapid detection and characterization of atmospheric NOx. There is also a need for the development of a method and apparatus for detecting trace vapors of NO and NO.sub.2, and for differentiating between the two species.